Bayer process for production of alumina trihydrate

ABSTRACT

An improvement in the Bayer process for producing alumina trihydrate in which bauxite ore is digested in a mixture with a digestion sodium aluminate liquor to produce a slurry, the slurry is settled to remove undigested residues therefrom and produce a pregnant liquor of sodium aluminate and alumina trihydrate is precipitated from the pregnant liquor in the presence of alumina trihydrate recycled as seed. When beginning the precipitation, an aliquot of the pregnant liquor with recycled alumina trihydrate is removed, subjected to mechanical action of an intensity and for a time sufficient to cause formation of seeds, and returned with the formed seeds to the beginning of the precipitation.

FIELD OF THE INVENTION

The invention relates to a process for manufacturing of aluminatrihydrate by precipitation in the presence of a seed consisting of apregnant sodium aluminate liquor derived from the BAYER process, inorder to separately control the sodium content and the particle sizedistribution of the precipitated alumina trihydrate, while maintaininghigh productivity.

The physicochemical characteristics of alumina trihydrate, in particularthe purity, particle size distribution, the size of elementarycrystallites and the degree to which they agglomerate in more or lessfriable grains, are parameters that control the properties of aluminahydrate and of the resulting alumina. These characteristics must beadapted as a function of product applications; metallurgical alumina forthe production of aluminum by igneous electrolysis or technical aluminasfor use in a wide variety of fields such as flame retarding fillers forplastics, refractory materials, abrasives, ceramics and catalysts.

These characteristics are interdependent in a conventional BAYER processof the American or European type, which makes it difficult to adapt thealumina perfectly to its final application. Thus, an alumina producerwho has a Bayer process with which he can make an alumina trihydratewhile adjusting the particle size distribution independently of theresidual sodium content will find that this manufacturing process giveshim a considerable advantage in terms of operating flexibility. Thisadvantage will be particularly appreciable if the performances of theprocess in terms of productivity and reliability are maintained, or evenimproved, compared with values obtained with known processes.

DESCRIPTION OF RELATED ART

The Bayer process is widely described in the specialized literature, andis the essential technique used for manufacturing of alumina fortransformation into aluminum by igneous electrolysis or for use in thestate of a hydrate, a transition alumina, calcinated alumina, sinteredor molten alumina, within a wide range of applications in theengineering aluminas field.

According to this process, the bauxite ore is digested when hot by meansof an aqueous solution of sodium hydroxide with an appropriateconcentration, thus causing extraction of the alumina and resulting in aslurry composed of particles of undigested residue in a sodium aluminatesolution called “aluminate liquor”.

In general, this slurry is then diluted to separate undigested residuesin the aluminate liquor, by settling. Once “cleaned”, this liquor iscooled to a temperature at which it is in a strongly unbalancedsupersaturated state. At this stage, it is called “pregnant liquor”. Theliquor saturation or stability state, is characterized by the followingratio by weight:$R_{p} = \frac{{concentration}\quad {of}\quad {dissolved}\quad {Al}_{2}O_{3}\quad \left( {{in}\quad g\text{/}l} \right)}{{caustic}\quad {concentration}\quad {Na}_{2}O\quad \left( {{in}\quad g\text{/}l} \right)}$

This strong unbalance results in the crystallization of aluminatrihydrate in a phenomenon called “precipitation”, which is unstable butreacts slowly, and it is essential that it must be perfectly controlled.After precipitation, the sodium aluminate liquor, depleted in aluminadue to the precipitation and called the “spent liquor”, is recycledafter concentration to ore digestion.

Precipitation is a complex phenomenon including crystallization, crystalgrowth and agglomeration of crystallites leading to the production ofparticles of alumina trihydrate. This phenomenon takes place slowly, sothat this pregnant liquor has to be circulated in a sequence of tankswith partial recycling of the trihydrate produced, this recycling beingdesigned to facilitate precipitation of crystallites with respect to theseeds or “nuclei”. The “aluminate slurry” is the medium that circulatesin the precipitation line; it is a mix of aluminate liquor and solidalumina trihydrate particles which have just precipitated or which areresulting from agglomeration or crystal growth of trihydrate particlesdrawn off near the downstream end of the precipitation line andreinjected at the upstream end of the precipitation line. Particlesrecycled due to their small size are called “fines”. In practice,precipitation is carried out differently in European BAYER lines and inAmerican BAYER lines.

American BAYER lines use an aluminate liquor with a lower causticconcentration; although the soda is less productive (less aluminatrihydrate is produced for the same volume of aluminate liquor) howeverit is rich in seeds. The added recycled trihydrate is a seed and aregulating makeup carried out at the beginning of precipitation, both inthe first step called “agglomeration” and in subsequent “feed” or“growth” steps.

European BAYER lines use an aluminate liquor with a much higher causticconcentration. This means that more trihydrate is obtained for a givenvolume (see below for the definition of productivity) but makesprecipitation more difficult since there are far fewer crystallizationseeds. In general, the pregnant liquor is mixed with seeds in a firsttank, called the seed tank, to form a “pregnant slurry”. Precipitationcontinues in subsequent tanks called “precipitators” either by feedingexisting grains or crystallites, or by the appearance of new seeds, orby the formation of particles by agglomeration of crystallites.

In the American and European processes, direct recycling from aluminatrihydrate produced proves to be inadequate when it is required to makea product with precise characteristics. U.S. Pat. No. 3,486,850 and U.S.Pat. No. 4,311,486 recommend that the trihydrate produced should besorted before recycling fines at several steps of the precipitation. InEuropean processes, simply recycling alumina trihydrate is notsufficient to give good control over the size and structure ofprecipitates. “Particle size crises”, in which the trihydrate producedis either too fine or too coarse, occur suddenly, and the remedy must beapplied very cautiously without any immediate effect—othercharacteristics of the trihydrate produced must not be disturbed, andthe productivity of the pregnant aluminate liquor during precipitationmust not be reduced.

It is known that the addition of seeds produced outside the BAYER linecan make process control more flexible. Thus, FR-A-2 709 302 proposesrecovering sorted fines from another precipitation line. FR-A-1 525 302proposes seeding of an auxiliary pre-calibrated seed in the form of anultrafines gel at the beginning of precipitation, and EP-A-0 344 469proposes ground trihydrate as an auxiliary pre-calibrated seed.

But these solutions are not economic. The solution in FR-A-1 525 302 isexpensive, and the solution recommended in FR-A-2 709 302 is onlyattractive if there is a second precipitation line on the sameindustrial site. Finally, the solution provided by EP-A-0 344 469 isvery expensive since it is necessary to obtain a very closely controlledparticle size distribution of the ground seeds, in order to obtain asatisfactory quality of alumina trihydrate.

Thus, even making use of prior art, the expert in the field has noreliable and inexpensive industrial process by which he can make atrihydrate, with independent control over the BAYER line operatingparameters, particularly with an influence on the particle sizedistribution and the residual sodium content, while maintaining goodproductivity.

Reliability includes precise and stable long term reproduction of thetarget characteristics starting from adjustment set values, thereforerequiring low dispersion of the characteristics obtained compared withthe target characteristics.

Productivity means productivity of the pregnant sodium aluminate liquorderived from alkaline digestion of bauxite using the Bayer process whichprecipitates during precipitation in the presence of a seed. The weightratio Rp characteristic of the saturation state of alumina dissolved inthe liquor in the Bayer cycle, can be used to determine the productivityof the liquor during the precipitation. This is defined by the quantityof alumina restored, in the form of alumina trihydrate, aftercrystallization of the pregnant liquor, compared with a given volume ofpregnant liquor. The productivity, expressed in kilograms of alumina percubic meter of liquor (kg of Al₂O₃/m³), is obtained by multiplying thevariation of Rp before and after precipitation by the causticconcentration of the pregnant liquor.

In general, this concentration is higher in European type Bayerprocesses (more than ≈140 g Na₂O/l) than in American type processes(≈120 g Na₂O/l) and this is why the precipitation productivity of thepregnant liquor is considered to be good when it exceeds 70 kg Al₂O₃/m³for an American type Bayer process and when it exceeds 80 kg Al₂O₃/m³for a European type Bayer process.

Particle size crises observed on European BAYER lines are related to thefact that the aluminate liquor is less pregnant, which prevents thepresence of a large quantity of seeds in the seed tank. American lines,currently less sensitive to the particle size crises described above,could eventually become more sensitive because international qualitystandards impose a reduction in the caustic content in the aluminaproduced, which implies that the degree of alumina supersaturation ofthe aluminate has to be reduced. Thus, introduction of a means ofgenerating seeds to control the particle size distribution and residualsodium content in the trihydrate independently is useful for all BAYERlines, of the American type and the European type, regardless of thesize of these installations.

SUMMARY OF THE INVENTION

The process according to the invention is based on the surprisingobservation that if intense mechanical action is applied to a part ofthe slurry at the beginning of precipitation, the formation of seeds ortrihydrate nuclei that act as seeds is observed several tens of hourslater. Several types of intense mechanical action were carried out,firstly by grinding with a vibrating grinder, and also by cavitation athigh frequency with a sonochemical device, and by intense shear of theslurry using a turbine or a centrifugal pump.

“Beginning of precipitation” means a step in which the location of thealuminate liquor is near the upstream end of the precipitation line, forexample in the seed tank or the first precipitation tank in a Europeantype BAYER line or in the agglomeration tank or preferably the firstfeed tank, in an American type BAYER line.

The various types of mechanical actions envisaged in this invention arecarried out on an aliquot of the slurry drawn off at the beginning ofprecipitation and returned into the Bayer circuit preferably into thesame tank or after the drawing off point, after the mechanical actionhas been applied. Depending on the mechanical action considered, thealiquot corresponds to a variable fraction of the slurry in circulationin the Bayer circuit; typically it represents 5 to 40% and preferablybetween 10 and 30% of the slurry for actions causing intense shear, andin much lower proportions (less than 1%) for grinding and application ofultrasounds.

The mechanical action may be grinding, preferably done by a vibratinggrinder, of part of the drawn of f slurry, which is then reinjected nearthe upstream end of the precipitation line.

The mechanical action may also be cavitation at high frequency obtainedby means of an ultrasound generator applied to part of the slurry beingrecirculated on a point near the upstream end of the precipitation line.

The mechanical action may also be caused by a device generating alaminar or turbulent flow in the slurry, within which there are largespeed gradients that could not be obtained by simple implantation ofrings or baffles counteracting the natural circulation of the slurry.

Seeds are probably created fairly early, but their size is such thatthey are undetectable with routine testing means, in this case aparticle counter, which can only detect particles if their diameterexceeds 0.5 to 1 μ. The applicant observed that if this action (whichhas no immediate effect) is applied continuously, after a few weeks itbecomes easier to control the size and morphology of the aluminatrihydrate particles. This type of continuous or regularly repeatedmechanical action avoids annoying “particle size crises” encountered inhigh productivity processes.

This phenomenon takes place very slowly, and the first effects onlyoccur after several tens of hours. The said effects take a particularlylong time to observe since part of the trihydrate produced is recycled.They result in:

either a gradual increase in the size of grains if the number of seedsadded is insufficient, a phenomenon which in the past was compensated byaction on the temperature of the slurry in the seed tank, with a directinfluence on the residual sodium content of the trihydrate produced,

or by stabilization of the size of particles produced by theprecipitation line, if the added seeds exceed a critical value; as thenumber of seeds added in this way increases, the equilibrium size ofparticles produced during the precipitation cycle reduces.

In order to clarify the situation, the applicant isolated part of thepregnant aluminate slurry at the beginning of precipitation and observedthe initial effects and the long term effects of various mechanicalactions produced in the said slurry, in an auxiliary tank.

This phenomenon was demonstrated in the laboratory by means of a specialdevice; a circuit connecting an auxiliary tank to the seed tank, withshear being applied in the auxiliary tank and which, aftersimultaneously stopping the pregnant slurry feed and the mechanicalaction, allows nuclei to increase in size sufficiently so that theirpresence can be detected. By accelerating observations of the results,this device could be used to compare different means of creating theshear and defining parameters creating conditions under which an optimumseed can be produced. It was also used to define a preferred device forimplementation of the invention under industrial conditions.

It is thought that the effect of introducing shear into the slurry isfirstly to separate a number of crystallites, or even particles, frompreviously formed large agglomerates. Incidentally, this shear may alsoincite secondary crystallization phenomena. Laboratory tests have shownthat the number of seeds generated is particularly high if the slurry issubjected to intense shear for a given time. The effect of thisgeneration only becomes significant (in other words has an influence onthe particle size distribution of the alumina trihydrate produced) abovea certain critical shear rate, and if it is maintained at least for agiven critical time. This critical shear rate and this critical timedepend on the means used to generate the mechanical action introducedinto the slurry.

Two of the tested mechanical means for generating seeds proved to beuseful:

a shear turbine operating at maximum speed but for a variable time,depending on the number of nuclei to be produced,

a centrifugal pump operating either on request, or continuously but witha variable flow depending on the number of nuclei to be produced. Thisis an additional pump added to the existing pump(s) that circulateslurry in the precipitation line. One preferred solution is to make abypass circuit at a tank at the beginning of precipitation (for examplethe seed tank) by which a high recirculation flow may be imposed on thepregnant aluminate slurry.

Concerning the shear turbine, turbulence and shear forces initiatingwithin the air gap between the stator and rotor have a controllinginfluence on the generation of seeds. The high frequency of mechanicalshocks on the stator-rotor system at high peripheral speeds creates avery strong dispersion of slurry particles. A significant particlegeneration effect is observed when the peripheral speed of rotor ribsexceeds 20 m/s. In the specific case of a shear turbine, the order ofmagnitude of shear speeds imposed on the slurry when it enters thestator-rotor air gap may be estimated at 10³-10⁴ s⁻¹.

The applicant observed that the amount of energy used can be reduced byperforming the shear on an aliquot of the pregnant slurry drawn off andreturned into the seed tank. The drawn of f aliquot is part of thealuminate slurry flow such that the number of seeds created and recycledin the seed tank is sufficient. For a given mechanical energy, theresulting shear intensity depends on the average circulation speed ofthe slurry; as the circulation speed reduces, it becomes easier to reachthis critical shear. furthermore, wear due to abrasion by slurryparticles is lower when the average circulation speed of the slurry islower.

Thus, one or more shear turbine(s) are either dipped directly into theseed tank, or may be mounted on one (or several) recirculationcircuit(s) on the seed tank, where it (they) acts (act) as a pump(pumps).

Concerning the centrifugal pump, the applicant observed that it wasadvantageous to place a recirculation circuit on the auxiliary tank atthe feed tank, which means that the recirculation flow is significantlygreater than the auxiliary tank feed flow. The critical shear is moreeasily achieved in this auxiliary tank, particularly if the high flowrecirculation pumps the slurry into the high shear area several times.With this type of circuit with a high recirculation flow on theauxiliary tank, itself fed by a slurry aliquot from the seed tankreturning to the seed tank at a lower flow rate, the generated hourlyparticle flow is significantly greater than what could be obtained witha shear turbine with the same power.

Sufficiently large pumps are found in the shops to feed a singlerecirculation circuit and to generate the required number of seeds. Thisis why, in the preferred embodiment of the invention, it is onlynecessary to insert a centrifugal pump into this recirculation circuit,with mechanical and hydraulic characteristics comparable to those of theexisting centrifugal pump, used to circulate the slurry in theprecipitation line. A second additional pump with lower mechanical andhydraulic characteristics, is used for return into the seed tank fromthe auxiliary tank.

If only one tenth of the slurry in the seed tank is drawn off to feedthe auxiliary tank, in which the recirculation flow is at least 5 timeshigher, the number of seeds generated and returned into the seed tank issufficient for the entire slurry entering the seed tank.

For grinding, the seed creation mechanism is different; grains arebroken and the highly active rupture surfaces are the source ofsecondary crystallization. In order to be able to control the number ofseeds created, it is preferable to act only on a very small fraction(less than 1% typically of the order of 1 per thousand) of the totalslurry circulating in the Bayer circuit precipitation line.

The preferred grinding device according to the invention is a vibratinggrinder comprising two grinding tubes which are forced to move in anapproximately circular rotation movement by vibrations caused byout-of-balance masses. The two tubes are usually filled to 65% bygrinding bodies. Pulses transferred to the grinding bodies through thetube walls generate impacts imposed on the slurry passing through thegrinder. Depending on the material to be treated and the requiredfineness, balls, bars, or even cylpeps (segments of bars) of differentsizes and made of different materials, may be used as grinding bodies.The use of steel cylpeps has proven to be beneficial within theframework of the invention.

The very low proportion of slurry to be treated makes it possible to userelatively low power devices (of the order of 20 kW) since they willonly need to treat slurry flows of the order of about 1 m³/h, whereasthe slurry flow in an industrial Bayer line is of the order of athousand m³/h.

The use of high power ultrasounds (typically 100 W/cm2 at 20-500 kHz)causes an acoustic cavitation phenomenon that also causes breakage ofgrains with the creation of very active rupture surfaces. As forgrinding, it is preferable to act only on a small proportion, typicallyless than 5%, of the slurry in circulation in the Bayer circuitprecipitation line. The slurry in recirculation passes throughan-auxiliary tank in which ultrasounds are generated.

Regardless of the type of mechanical action used, it is useful to add anauxiliary tank in the circuit through which the slurry aliquot to betreated passes. In all these cases, it has been shown that it is usefulto add an alumina supersaturated liquor to the slurry in the auxiliarytank, for example an aliquot of the pregnant liquor drawn off at thevery beginning of the precipitation. Firstly, this adding dilutes solidscontent and the resulting lower viscosity facilitates operatingconditions, and also and especially significantly increases the Rp ofthe aluminate slurry and thus improves the efficiency of finesgeneration.

BRIEF DESCRIPTION OF THE DRAWINGS

The process according to the invention will be better understood afterreading the detailed description of its implementation.

FIG. 1 diagrammatically shows the precipitation steps in a European typeBAYER line.

FIG. 2 shows a bypass circuit on the seed tank, in which an aliquot ofthe pregnant aluminate slurry is circulated by means of a shear turbinethat acts as a recirculation pump. This turbine generates high shearaccording to the invention, in the pregnant aluminate slurry.

FIG. 3 shows an auxiliary tank fed by a recirculation circuit startingand ending at the seed tank, part of the aliquot passing through theauxiliary tank itself being extracted and recycled with high flow on theauxiliary tank by means of a centrifugal pump.

FIG. 4 diagrammatically shows a European type BAYER line with amechanical seed generation system according to the invention, in bypasson the seed tank.

FIG. 5 diagrammatically shows an American type BAYER line with amechanical fines generation system according to the invention, in bypasson the first growth tank.

FIG. 6 shows a circuit in bypass on the seed tank, in which there is acirculation of an aliquot of the aluminate slurry that passes through avibrating grinder.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples are based on a preferred embodiment of theinvention applied to European type BAYER lines with high precipitationproductivity, as shown on the diagram in FIG. 1. The pregnant aluminateliquor 1, with an Rp close to 1.2 and a caustic concentration between140 and 180 g of Na20/l, is mixed with alumina trihydrate 2 recycled atthe end of precipitation and impregnated with crystallized liquor. Thepregnant slurry 3 thus formed has an Rp close to 1.0. After 15 minutesresidence in the seed tank A, the slurry 4 is entrained under the effectof pump PC to the series of precipitators D1, D2, D3, . . . , Dn. A part7 of the slurry is separated from the crystallized slurry 6 so as toobtain classified alumina trihydrate 8 by classification C, to be usedin the final treatment (washing, calcination, etc.). The other part 9 isfiltered F, the filtered crystallized liquor 10 is further concentratedfor recycling for further digestion of the bauxite, whereas the filtrateis alumina trihydrate 2 used as a seed and mixed with the pregnantaluminate liquor 1.

To demonstrate the influence of high shear imposed within the slurry,the applicant made a small experimental device reproducing an auxiliaryseed tank fed with pregnant aluminate slurry 3 from a European BAYERline satisfying this description.

EXPERIMENTAL DEMONSTRATION

The experimental device was used to compare the variation in thecrystallization of pregnant aluminate liquor to which mechanical actionhad been applied (the test tank) with the crystallization of the samepregnant aluminate liquor to which mechanical action had not beenapplied (reference tank). These two 1 m³ tanks are fed continuously andin parallel at the same flow, with a pregnant aluminate slurry 3 withRp=0.96 and containing 155 g of Na₂O/l originating from the seed tank ina BAYER line. In these two tanks, the slurry is continuously stirredmechanically and is kept at a temperature of 60° C. The feed time issignificantly greater than the residence time of the slurry in thetanks, to ensure that the tanks are uniformly mixed.

The pregnant aluminate slurry in the test tank is subject to shear withvariable intensity and for a variable duration, the two tanks alwaysbeing fed. The feeds into the two tanks are then cut off at the sametime as the mechanical action is stopped.

The tanks are then isolated and kept stirred and at high temperature sothat crystallization can take place. Samples are taken at regular timeintervals to monitor changes in the particle size distribution and theprecipitation dynamics. Due to the measurement range of the analysisinstruments (particle counters), very fine particles smaller than 1 μmcannot be detected, and these particles can only be detected andquantified after crystal growth.

Tests with a Shear Turbine

We used an ULTRA TURRAX (registered trade mark) shear turbine operatingat 2900 rpm with a dissipated power of 6.3 kW. The maximum sheargenerated by the turbine is close to the peripheral part of the mobilepart, the speed at the periphery reaching 23 m/s. The turbine dippeddirectly into the test tank. Several shear durations were tested varyingfrom 0.075 to 3 times the average residence time of the slurry in thetest tank.

The results were expressed as the ratio of the nucleation frequency inthe test tank and in the reference tank. Starting from a shear durationequal to 0.375 times the average residence time of the slurry in thetest tank, the turbine generated ultra fines increasing in size after 48hours into fines (with an average diameter of between 1 and 2 μ)detectable by the particle counter. The number of these ultra finesincreases with the shear duration and is between 10 and 80 times greaterthan the number of ultra fines created without prior shear.

Regardless of the tested shear time, the hourly flow of fines generatedby this shear is approximately constant. In this case it was measured atbetween 3.5+1 and 4.5±1 10¹³ particles per hour.

Tests with a Centrifugal Pump

We used a SCHABAYER M40 centrifugal pump operating in a closed circuiton a bypass on the test tank itself. This pump operated for asignificantly longer time than the average residence time of the slurryin the test tank. The ratio between the pump recirculation flow and thetank feed flow was varied between 1 and 15.

With a ratio between the pump recirculation flow and the tank feed flowequal to about 10, the hourly flow of generated particles was measuredat 3.5±0.5 10¹⁴ particles per hour.

It is difficult to specify the shear rate reached within the slurry whenit is generated by the centrifugal pump. But apparently the shear effectis determined by the linear speed of the slurry at the entrance to thedisk blades; this speed depends partly on the mechanical and hydrauliccharacteristics of the pump, and also on the ratio between the pumprecirculation flow and the tank feed flow. Shear speeds imposed by thepump on the slurry are probably lower than the speeds generated by theshear turbine, but high flow recirculation enables the slurry to pass inthe high shear zone several times before returning to the seed tank.When the ratio between the recirculation flow and the feed flow exceeds5, it is found that generation of fines is sufficient to start to havean influence on the particle size in the slurry.

Tests with a Vibrating Grinder

We used a PALLA (registered trade mark) vibrating grinder with its twocylinders filled to 65% with steel cylpeps. The overflow level of theupper cylinder was adjusted to the maximum so that the grinder operatespractically full (maximum residence time for a given flow). The grindingload used is as follows:

upper cylinder 120 kg of cylpeps made of 16 mm steel

lower cylinder 120 kg of cylpeps made of 12 mm steel

All tests were carried out with the following operating parameters:

Flow (l/h) 300 100 300 Vibration amplitude (mm)  8  8  12 Residence timein the grinder (minutes)  8  24  8

The vibration amplitude on a scale of 6 to 12 mm, corresponds to thediameter of the circular envelope of the trajectory of a point on thevibrating wall.

Regardless of the tested vibration amplitude, the vibrating grindercreated many fines and considerable breakage of the hydrate grains. Thehourly flow of fines generated by this grinding is approximatelyconstant. In fact, values of between 5.6 and 21.5 10¹⁴ particles perhour were measured, depending on the grinder operating parameters, themaximum being obtained with 300 l/h and an amplitude of 12 mm.

Tests with Ultrasounds

Particle counts and a particle size distribution analysis were carriedout on a reference slurry and on the same slurry after an ultrasoundtreatment was applied with 100 W/cm2 at a frequency of 20 kHz for 30minutes.

After the ultrasound treatment, we observed a very large quantity offine particles with an average diameter D50 below 15 μm. The ratio ofthe number of particles in the treated slurry and in the referenceslurry is between 40 and 60 for particles with a diameter less than 6μm.

INDUSTRIAL EMBODIMENT

Shear Turbine

FIG. 2 shows a circuit Q in bypass on seed tank A, in which an aliquot11 of the pregnant aluminate slurry is circulated by a shear turbine Tthat acts as a recirculation pump.

This turbine generates a high shear according to the invention in thealiquot 11 of the pregnant aluminate slurry 3. Since the generation ofseeds is fairly low, several circuits of this type are made inrecirculation on the seed tank, since the total flow recirculating inthese circuits should be comparable with the slurry flow passing throughthe seed tank towards the precipitators.

Centrifugal Pump

FIG. 3 shows an auxiliary tank B fed by a recirculation circuit Qstarting and ending at the seed tank A. When valve V is opened, thealiquot 21 of the pregnant aluminate slurry 3 is extracted (21 a) fromthe seed tank A and is fed into auxiliary tank B. Part of this aliquot21 is itself extracted and recycled (22) in a recirculation circuit Q′on the auxiliary tank B by means of a centrifugal pump P. The aliquot 21b is drawn off from auxiliary tank B and directed into seed tank A bymeans of centrifugal pump PA. The flow of the aliquot extracted 21 a andpoured 21 b into the seed tank A is equal to about one tenth of the flowof aluminate slurry in the line. The centrifugal pump P circulates part22 of this aliquot 21 in the bypass circuit with a flow more than fivetimes higher.

An advantageous application of the invention consists of also feeding analiquot 1 b of the pregnant aluminate liquor 1 into the auxiliary tankB. The mix thus formed in the auxiliary tank B is then in a morepregnant state. This type of addition encourages precipitation andreinforces the generation of nuclei, by increasing the value of Rp ofthe slurry 22, part of which is subjected to a high recirculation flow.

FIG. 4 shows the device in FIG. 3 adapted to the circuit in the Europeantype BAYER line shown in FIG. 1. The pregnant aluminate liquor 1, withRp≈1.2 and with a caustic concentration of 140 g Na₂O/l, is mixed withrecycled trihydrate 2 impregnated with crystallized liquor. The slurry 3resulting from this mix enters seed tank A with an Rp close to 0.95.Part 21 of the slurry 3 passes through an auxiliary tank B to berecycled (21 b) on the seed tank A. An aliquot of the aluminate liquor 1(not shown in FIG. 4 to make the drawing more easily readable) mayoptionally feed auxiliary tank B without passing through the seed tank.A loop circuit is connected to the auxiliary tank B, in which part 22 ofthe slurry circulates with a higher imposed flow than the auxiliary tankfeed flow. The recirculation flow is imposed by a centrifugal pump Pthat operates continuously such that the ratio between the recirculationflow and the feed flow is kept at a value of between 5 and 15.

The temperature in the seed tank and in the auxiliary tank is kept at60° C., and then decreases in subsequent tanks. After three weeks, it isfound that the size of D50 grains stabilizes at a value of between 60and 90 μ depending on the chosen ratio of recirculation and feed flows.

FIG. 5 shows the adaptation considered for an American type Bayercircuit in which the device in FIG. 3 is mounted in bypass on the firstfeed tank N1, fed with the slurry originating from the agglomerationtank AG and mixed with the secondary seed 32 recycled from the secondarythickener ST. The hydrate produced 8′ exits from the primary thickenerPT in underflow. The crystallized liquor 10′ exits from the tertiarythickener TT in overflow, while the tertiary seed 33 exits from thetertiary thickener in underflow to be mixed with the secondary seed 32and then with the pregnant liquor 1′, the slurry 3′ obtained then beingpoured into the agglomeration tank AG. As shown in FIG. 3, it isadvantageous, particularly when a circuit is made in bypass on the firstfeed tank N1, to also feed the auxiliary tank B with an aliquot ofpregnant aluminate liquor 1′. This can significantly increase the Rp ofthe aluminate slurry, which is already less pregnant at this stage.

Vibrating Grinder

FIG. 6 shows a circuit Q in bypass on the seed tank A. An aliquot 21 aof pregnant aluminate slurry 3 is drawn off using a pump PA and a valveV. The slurry is directed towards a vibrating grinder BV working underpressure. The slurry 21 b outlet from the grinder is sent to the seedtank. The flow (of the order of 2 m³/h) is measured by a flow meter or acalibrated vessel placed at the grinder outlet. The flow is adjusted bymeans of a pinch valve. The excess drawn off slurry 21 c is directlyreintroduced into the slurry 4 which feeds the first precipitation tank.

ADVANTAGES OF THE PROCESS ACCORDING TO THE INVENTION

The process according to the invention can generate seeds within theslurry in controlled quantities and without involving BAYER lineoperating parameters which have an influence on the final productquality, for example the residual caustic content.

This process is useful for all processes in which high productivity ofthe aluminate liquor is important and in which the caustic content ishigh (greater than ≈140 g Na20/l).

What is claimed is:
 1. In a Bayer process for producing aluminatrihydrate comprising digesting bauxite ore in a mixture with adigestion sodium aluminate liquor to produce a slurry, settling theslurry to remove undigested residues therefrom and produce a pregnantliquor of sodium aluminate, and precipitating alumina trihydrate fromthe pregnant liquor in the presence of alumina trihydrate recycled asseed, the improvement comprising, at the beginning of saidprecipitation, removing an aliquot of the pregnant liquor havingrecycled alumina trihydrate, subjecting said aliquot to mechanicalaction of an intensity and for a time sufficient to cause formation ofseeds, and returning said aliquot having the formed seeds to thebeginning of the precipitation.
 2. Process according to claim 1, whereinthe precipitation is carried out in a Bayer line which includes a seedtank and precipitators, and the aliquot is taken from and returned tothe seed tank.
 3. Process according to claim 1, wherein theprecipitation is carried out in a Bayer line which includes anagglomeration tank and feed tanks, and the aliquot is taken from andreturned to the agglomeration tank.
 4. Process according to claim 1,wherein the precipitation is carried out in a Bayer line which includesan agglomeration tank and feed tanks, and the aliquot is taken from andreturned to a first feed tank.
 5. Process according to claim 1, whereinsaid mechanical action generates shearing in the suspension, with amaximum deformation speed exceeding 10³ s⁻¹.
 6. Process according toclaim 5, wherein the shearing is generated in said aliquot by a shearingturbine.
 7. Process according to claim 1, wherein the mechanical actionis produced by a method step selected from the group consisting ofgrinding, ultrasound cavitating, causing laminar flow and causingturbulent flow.
 8. Process according to claim 1, wherein the mechanicalaction is produced by a centrifugal pump.
 9. Process according to claim1, wherein the mechanical action is produced by introducing a means forproducing mechanical action directly into a tank containing suspensionused at the beginning of the digestion.
 10. Process according to claim1, wherein the mechanical action is produced by disposing a means forproducing mechanical action in a recirculation circuit.